1. Field of the Invention
The invention relates to a regulated charge pump, and particularly to a pulse width modulated charge pump.
2. Discussion of the Related Art
Charge pumps are well known in the art. For example, FIG. 1A illustrates a simplified charge pump 100 including an oscillator 101 that connects switch 104 to the ground GND terminal and switch 105 to the input voltage VIN terminal during the first half of a clock cycle. In this configuration, capacitor 102 (i.e. a pump capacitor) is charged to the input voltage VIN, whereas capacitor 103 (connected only to ground) is not charged. During the second half of the clock cycle, oscillator 101 connects switch 104 to the input voltage VIN terminal and switch 105 to the output voltage VOUT terminal. In this configuration, with the negative terminal of capacitor 102 connected to the input voltage VIN terminal, its positive terminal is increased to twice the source voltage. In this manner, capacitor 103 (i.e. a reservoir capacitor) is then charged to twice the input voltage. In this manner, charge pump 100 can produce an output voltage of 2VIN.
Although charge pump 100 is relatively easy to implement, its output voltage can undesirably drop when subjected to a load and can vary with its input voltage VIN. Moreover, some applications may require a voltage higher than the input voltage, but less than double. For example, a system may provide an input voltage of 3.3V, but need only 5.0V. FIG. 1B illustrates a known, modified charge pump 110 that could be used to address these issues. Specifically, charge pump 110, in addition to the components described in detail in reference to FIG. 1A, also includes a standard linear regulator 111 that can be used to regulate down the generated 2VIN to a specific intermediate output voltage.
Both charge pumps 100 and 110 require a system capable of handling the multiplied output voltage, e.g. 2VIN. Thus, special circuits may be needed to protect low-voltage transistors/devices from this multiplied output voltage. Additionally, linear regulator 111 in charge pump 110 requires significant silicon area for implementation, thereby undesirably increasing system cost.
FIG. 2 illustrates one example of a continuously modulating regulator 206 that includes a comparator 201 for responding to the difference between the output voltage 202 of a charge pump 200 and a reference voltage 203. Comparator 201 forms part of an analog feedback loop that can control the charging of a pump capacitor 204, which in turn can control the charging of a reservoir capacitor 205. U.S. Pat. No. 5,680,300 describes this regulator in further detail. Unfortunately, regulator 206 provides a poor efficiency and undesirably increases the IC topology by using large n-channel devices.
Alternatively, replacing either the linear regulator or the continuously modulating regulator, a burst mode regulator for a charge pump can turn a clock on and off as needed (wherein, when the clock is on it runs at a fixed frequency) to bring the output voltage to the desired voltage level. One known charge pump providing such burst functionality is the MAX1682 device sold by Maxim, Inc. However, turning on and off the clock can cause some inter-modulation problems with other circuits on the board. For example, inter-modulation could cause an error signal or an undesirable feedback signal (such as an audible sound in a cell phone application).
Therefore, a need arises for a charge pump that provides an intermediate output voltage without requiring significant silicon area or causing inter-modulation problems.
A charge pump system and method including a charge pump and a pulse width modulated controller are provided. The charge pump includes a pump capacitor, a reservoir capacitor, and pump circuitry. During a first phase, the pump circuit couples the pump capacitor between a first supply voltage and a second supply voltage. During a second phase, the pump circuit couples the pump capacitor and the reservoir capacitor in series between the first supply voltage and an output terminal of the charge pump system. The PWM controller, which is coupled to the pump circuitry, determines the phase of the charge pump.
In one embodiment, the pump circuitry includes a feedback loop to the PWM controller, and the PWM controller includes an error amplifier that compares a voltage on the feedback loop to a reference voltage. The PWM controller can further include a comparator for receiving an output of the error amplifier and a ramping signal, thereby generating a PWM signal. A multiplexing circuit can select the PWM signal or a switching signal, wherein the output of the multiplexing circuit determines the phase of the charge pump. Specifically, this output determines a first time associated with the first phase and a second time associated with the second phase. In a preferred embodiment, the charge pump provides xe2x80x9cbreak before makexe2x80x9d switching, thereby preventing inadvertent discharge of the pump capacitor in the first phase.
A circuit and method for selectively isolating a first line from a second line are also provided. The circuit can include first, second, and third transistors. The first transistor has a first control terminal, a first current-carrying terminal, a second current-carrying terminal, and a first body. The second transistor has a second control terminal, a second source, a second drain, and a second body, wherein the second source is connected to the first current-carrying terminal and the first line. The third transistor has a third control terminal, a third current-carrying terminal, a third current-carrying terminal, and a third body, wherein the third source is connected to the second current-carrying terminal and the second line, the third drain is connected to the second drain, and the first, second, and third bodies are connected to the second drain.
The isolating circuit further includes control circuitry coupled to the first, second, and third control terminals. During a first phase, in which a first voltage on the first line is greater than a second voltage on the second line, the first and third transistors are turned off, the second transistor is turned on, and the first current-carrying terminal functions as a source. During a second phase, in which the second voltage on the second line is greater than the first voltage on the first line, the first and third transistors are turned on, the second transistor is turned off, and the second current-carrying terminal functions as a source.